Lecture Notes: Chapter 17 - The Endocrine System

Overview

• Transition from the nervous system to the endocrine system.
• Focus on the integration between the nervous system and the endocrine system.

Endocrine System

• Composed of glands or glandular structures.
• Secretes hormones into the bloodstream (endocrine glands) as opposed to exocrine glands, which secrete externally.
• Hormones are chemical messengers.

Communication in the Body

• Nervous System: Uses electrical signals via neurons for fast communication.
• Endocrine System: Uses hormones for communication, influencing distant tissues through the bloodstream.

Modes of Chemical Signaling

1. Direct Communication: Cells directly adhere and share cytoplasm through gap junctions.
2. Paracrine Communication: Local communication through extracellular fluid; involves paracrine factors.
3. Endocrine Communication: Hormones travel through the bloodstream to distant target cells.
4. Synaptic Communication: Neurotransmitters cross synapses between neurons.
5. Autocrine Communication: Cells respond to their own secretions.

Nervous vs. Endocrine Systems

• Speed: Nervous is fast; endocrine is slow due to hormone circulation.
• Duration: Nervous has short impacts; endocrine has long-lasting effects.
• Modulation: Nervous uses frequency modulation; endocrine uses amplitude modulation.
• Impacts: Nervous impacts are specific; endocrine impacts are widespread.

Hormone Structural Categories

1. Amino Acid Derivatives: Modified amino acids (e.g., tyrosine, tryptophan).
   • Examples: Thyroxine, catecholamines, melatonin.
2. Peptide Hormones: Chains of amino acids (e.g., glycoproteins, small proteins).
   • Examples: Insulin, growth hormone.
3. Lipid Derivatives: Derived from lipids (e.g., eicosanoids, steroids).
   • Examples: Testosterone, estrogen, prostaglandins.

Hormone Solubility

• Polar (water-soluble) hormones bind to membrane receptors.
• Nonpolar (lipid-soluble) hormones can cross membranes directly.

Hormone Circulation

• Freely Circulating: Rare, quick removal; half-life ~30 minutes.
• Bound to Plasma Proteins: Common, longer half-life (days/weeks).

Patterns of Hormone Secretion

1. Chronic Secretion: Constant rate of release (e.g., thyroid hormones).
2. Acute Secretion: Released in response to stimuli (e.g., insulin).
3. Episodic Secretion: Occurs at specific intervals (e.g., menstrual hormones).

Control of Hormone Secretion

• Humoral: Response to local conditions (paracrine factors).
• Neural: Direct stimulation by the nervous system.
• Hormonal: Controlled by other hormones (hormone pathways).

Hormone Pathways

• Releasing Hormones: From hypothalamus, trigger tropic hormones.
• Tropic Hormones: From anterior pituitary, stimulate other glands.
• Terminal Hormones: Produced by target endocrine glands.

G Protein and Second Messengers

• Membrane-Bound Receptors: Use G proteins to activate intracellular pathways.
• Second Messengers: Cyclic AMP or calcium/calmodulin complexes.
• Intracellular Receptor Binding: Nonpolar hormones bind to internal receptors affecting gene expression.

Hormone Action on Cells

• Steroid Hormones: Directly enter cells and impact gene expression.
• Thyroid Hormones: Transported into cells, alter metabolism and gene expression.

Summary

• The endocrine system provides long-lasting and widespread modulation of body processes through hormones.
• Understanding hormone pathways and the interplay between different systems is critical for comprehending body regulation.